29 Jun 2021

Raspberry fertilization: theory and best practises


Fertilisation is the practice of modifying the chemical characteristics of the soil in order to provide the plant with the elements it needs to be able to vegetate and develop in the best possible way.

The raspberry has an intense vegetative activity which, for large parts of the growing season, leads to the simultaneous development of both vegetative elements (leaves, new shoots, roots) and generative elements (flowers and fruit).
In order to do all this, the plant uses what it can capture with its roots and leaves: water, air, solar energy and nutrients.

In normal raspberry cultivation we remove leaves and shoots from the raspberry orchard as well as fruit(Fig 1 & 2). These parts of the raspberry cost the plant energy and use the nutrients it has managed to absorb.

Figures 1 and 2 Raspberry spring pruning and harvesting: with these operations, several kilograms of material are removed from each plant, which is equivalent to several tens of tonnes per hectare. SOURCE: Molari Berries & Breeding

The idea is therefore to provide our raspberry plot with the equivalent of the nutrients used to produce those shoots and raspberries through fertilisation. And more specifically, the idea must be to provide the precise amount of nutrients needed to ensure that the plant can develop to its full potential, to guarantee good fruiting and, in the case of soil cultivation, to avoid reducing the fertility of the soil in which the raspberry is grown.

Both fertilisers and manures can be used, with the aim of providing the right levels of different elements, generally grouped into:
- macro elements (those used in larger quantities): Nitrogen (which we can find indicated by its symbol N), Phosphorous (P), Potassium (K)
- meso elements (used in intermediate amounts): Magnesium (Mg), Calcium (Ca) and Sulphur (S)
- micro elements (used in small amounts by the plants): Iron (Fe), Manganese (Mn), Boron (B), Molybdenum (Mo).



The macro elements also include Carbon (C), Oxygen (O) and Hydrogen (H), which the plant obtains from the air (carbon and oxygen) and water (hydrogen), of which plants are more than 90% composed. These are essential elements for plants, but which we do not provide through fertilisation. The remaining 10% or so of plants are made up of the mineral part, which is precisely the part that we can provide with fertiliser.

Figure 3 Plant nutrition from air and soil. SOURCE: Anka Rojc Polanec et al., 2014, 'Fundamentals of plant nutrition and fertilisation'.

It should be noted that the terms 'macro, meso, micro elements' refer only to the total amount that is used by plants for each of these elements.

It is also important to stress that this categorisation is in no way intended to be a ranking of the importance of each of these elements for the growth of the plant itself.


An important concept to always bear in mind is that if there is even a deficiency, i.e. a lack of even one of the nutrients, while all the other elements are present and available in ideal quantities, the plant suffers and is said to go 'deficient'. It is therefore important to be balanced and not to forget any of the plant's nutrients.

This is a principle of agronomy, also known as Liebig's law, exemplified by the well-known image of Liebig's barrel(Fig. 4): growth is controlled not by the total amount of natural resources available, but by the availability of the scarcest.

Figure 4 Liebig barrel. SOURCE: en.garynevillegasm.com

That is: if, as in the case of the picture, nitrogen (N) is the element present in the smallest quantity, all plant development will be limited by the presence of nitrogen, even if the other elements are present in larger quantities.

By the same principle, elements that are contributed disproportionately more than the others will be wasted, as the plant will not be able to use them anyway.


NitrogenIt is a fundamental element in the constitution of plant proteins and its presence has a significant influence on plant growth and consequently on production potential. On the other hand, its excess can lead to a vegetative imbalance to the detriment of fructification and also to greater susceptibility to winter frost damage. Nitrogen can be absorbed in both ammoniacal and nitric form(see Fig. 5). It can also be absorbed via the leaves.

PhosphorusWe find it in proteins and in general in any metabolic process requiring the use of ATP (adenosine triphosphate) which is the "energy currency" used by the plant. It is not a very mobile element in the soil, so it may well be present in large quantities, but not all of it is available to the plant. Especially at times of low activity, such as when there is excess water or cold weather, phosphorus may not be easily absorbed. Phosphorus is absorbed in the form of the phosphate ion.

PotassiumPotassium is very important for fruit quality and the plant's resistance to stress. A good level of potassium ensures optimum fruit enlargement and better storability, as well as greater resistance of the cell walls. It is absorbed as K+.

Magnesiumis a key component of the chlorophyll molecule and active in several enzymatic reactions. It is absorbed as Mg++.

Calciumis the main element in cell walls, essential for cell growth. It is absorbed as Ca++.

MicroelementsThere are many functions of micro-nutrients in normal plant development and it would be difficult to reduce the description to a few lines. We suggest a separate topic.

Figure 5 Nutrients and their forms of absorption.


Fertilisers are those compounds that contain the nutrients that plants are actually able to absorb.

Fertilisation can be done with organic and mineral fertilisers.

Organic fertilisers are, for example, manure, and in general animal faeces, which provide the soil with organic matter and nutrients. They are normally slower-releasing and for this reason can be preferred in the pre-planting fertilisation phase, when the crop is not yet present.

Mineral fertilisers, on the other hand, are quicker acting and are produced from an inorganic base.

Organic and mineral fertilisers have both advantages and disadvantages and should not be seen as alternatives to each other. Their characteristics should be studied and both should be used appropriately to achieve the best results.

The recommended mineral fertilisers are:
- nitrogen: nitro ammoniacal form. In the case of basic pH soils, the use of acidic fertilisers such as sulphate and ammonium phosphate is recommended(Fig. 6).
- phosphorus: ammonium phosphate.
- potassium: potassium sulphate.
- calcium: calcium nitrate.
- magnesium: magnesium sulphate.
- trace elements: iron and manganese chelates.

Figure 6 Ammonium sulphate.

Let us now give a practical example of how to calculate the amount of fertiliser to be spread.

Normally, we express our plants' need for a certain element in kg of that specific element. So if we need to supply 80 kg of nitrogen (80 kg of the element nitrogen) and we have a fertiliser such as ammonium sulphate available, which contains 21% N (the amount we can read on the label), this means that we will have to supply 380 kg of ammonium sulphate. In fact, 21% of 380 kg of ammonium sulphate is equivalent to the 80 kg of nitrogen that we need to supply to our plant.


It is important to provide fertilisers, but it is equally important to make them available. That is, it is necessary to create an environment in which the fertilisers can actually be absorbed by the plant.

We have to prepare the growing environment for our plants before planting: this is something we cannot do after transplanting. If the right conditions are not in place from the outset we risk wasting our efforts and investments. This should be taken into account when preparing the plant.

As we know, there are plants that prefer a neutral pH and other plants that prefer a sub-acidic pH. This is the case with the raspberry, which prefers a pH of around 6. The raspberry is able to efficiently absorb the nutrients it needs, in balanced quantities, at a pH of around 6.

Looking at Figure 7, a pH that is too low would make elements such as nitrogen, phosphorus, calcium and others unavailable. A pH that is too high, on the other hand, would excessively reduce the ability to absorb iron, manganese and in general especially trace elements.

Figure 7 The influence of soil pH on nutrient availability. SOURCE: http://www.ekogrow.it

Not to be underestimated is the interaction between micro organisms in the soil and the ability of plants to absorb nutrients: a good presence of mycorrhizal fungi can promote the absorption of certain elements, such as phosphorus.


Excesses of one element can lead to a more limited uptake of another element and this leads to a deficiency. We think that we have fertilised enough, but due to an imbalance we do not make all the elements available. For example: an excess of potassium (K) leads to a limited uptake of magnesium (Mg) and boron (B), but conversely favours an over-absorption of iron (Fe) and manganese (Mn).

All this leads to imbalances, which are to be avoided. As in all things: balance is needed.



We always suggest regular analyses of the soil, irrigation water, plant leaves and, if fertigation is carried out, routine analyses of the nutrient solution to check that the system is working properly. These analyses will allow us to study the ideal fertilisation plan for the plant.

It would be advisable to study soil and foliar analyses at the same time, because the former give us an idea of what is present in the soil, the latter give us an idea of the actual capacity of the plant to use what they have available in the soil.

Constant and punctual observation of the plants is very important because it is they who tell us how they are doing. In particular we advise you to keep an eye on the signals given by our raspberries with the colouring of their leaves: FIG 8.

Figure 8 Visual diagnosis of nutritional deficiency. SOURCE: http://www.smart-fertilizer.com


Plants are able to take up nutrients both from the roots and the leaves.
Normally macro and meso elements are administered by the roots, while micro elements can also be applied by the leaves, making them more readily available.

In the case of soil cultivation, as far as maintenance fertilisation is concerned, if a fertigation system is not available, fertilisers are spread in stages throughout the year.

Normally, the more washable elements such as nitrogen are distributed a couple of times during the year, splitting the fertilisation into two periods, before the vegetative awakening and before flowering.

In the case of phosphorous and potassium, on the other hand, as these elements are not very mobile in the soil, it is better to bury the fertiliser in the autumn, in anticipation of the following growing season, and in any case it is essential to have prepared the pre-planting groundwork well. Do not expect immediate results after fertilisation, as is the case with nitrogen: if necessary, it is better to use foliar fertilisers to remedy a deficiency more quickly.

We recommend the fertigation solution because it makes it easier to manage both irrigation and fertilisation of the plants in a timely manner.

In the case of soilless growing, we consider it indispensable because in this way we are able to manage both fertilisation and irrigation at the same time. Certainly the maximum potential we can achieve with a good fertigation system is greater than if we were to fertilise manually several times a year.


Different phenological phases require different fertilisation. The individual nutrients have different importance depending on the period of plant development. In general, nitrogen is required to a large extent during the vegetative development phase. Less in the other phases.(Fig. 9)

Figure 9 Raspberries in vegetative growth phase. SOURCE: Molari Berries & Breeding

We recommend phosphorus fertilisation especially in the phases when root growth is most active(Fig. 10).
Potassium and calcium, on the other hand, are required in abundance during the fruiting phase, and specifically in the fruit growth phase before ripening.

Figure 10 Raspberry roots in rapid growth phase. SOURCE: Molari Berries & Breeding

Generally speaking, the raspberry needs the following nutrients throughout the year:
- Nitrogen: up to 80 kg N per hectare per year.
- Phosphorus: up to 60 kg of P per hectare per year.
- Potassium: up to 100 kg K per hectare per year.
- Magnesium: up to 100 kg of Mg per hectare per year.

These figures should be taken with great caution and we recommend that you always seek the advice of an experienced technician if you have no experience with raspberry cultivation.

Fertilisation is not the only production factor that can determine the success or failure of our crops. However, its importance affects about 30% of the results we can obtain: good and precise fertilisation is certainly decisive in maximising the yield of our plant.

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